I am indebted to the Editors of the _Nineteenth Century_ and of the _Fortnightly Review_ for permission to reprint such of the following Essays as have appeared in the pages of those periodicals: and so large a proportion of the papers has been published in the _Nineteenth Century_ that my acknowledgments are especially due to Mr. Knowles. T. H. H. _May 4, 1892._ * * * * * {1} PROLOGUE Le plus grand service qu'on puisse rendre à la science est d'y faire place nette avant d'y rien construire.--CUVIER. Most of the Essays comprised in the present volume have been written during the last six or seven years, without premeditated purpose or intentional connection, in reply to attacks upon doctrines which I hold to be well founded; or in refutation of allegations respecting matters lying within the province of natural knowledge, which I believe to be erroneous; and they bear the mark of their origin in the controversial tone which pervades them. Of polemical writing, as of other kinds of warfare, I think it may be said, that it is often useful, sometimes necessary, and always more or less of an evil. It is useful, when it attracts attention to topics which might otherwise be neglected; and when, as does sometimes happen, those who come to see a contest remain to think. It is necessary, when the interests of truth and of justice are at stake. It is an evil, in so far as controversy always tends to degenerate into quarrelling, to swerve from the great issue of what is right and what is wrong to the very small question {2} of who is right and who is wrong. I venture to hope that the useful and the necessary were more conspicuous than the evil attributes of literary militancy, when these papers were first published; but I have had some hesitation about reprinting them. If I may judge by my own taste, few literary dishes are less appetising than cold controversy; moreover, there is an air of unfairness about the presentation of only one side of a discussion, and a flavour of unkindness in the reproduction of "winged words," which, however appropriate at the time of their utterance, would find a still more appropriate place in oblivion. Yet, since I could hardly ask those who have honoured me by their polemical attentions to confer lustre on this collection, by permitting me to present their lucubrations along with my own; and since it would be a manifest wrong to them to deprive their, by no means rare, vivacities of language of such justification as they may derive from similar freedoms on my part; I came to the conclusion that my best course was to leave the essays just as they were written;[1] assuring my honourable adversaries that any heat of which signs may remain was generated, in accordance with the law of the conservation of energy, by the force of their own blows, and has long since been dissipated into space.